At the end of my Pacific Coast trip, this was my carry on luggage to take on the train. For a two day train ride, your carry on is more than you typically would take a a carry on.
I have used those mesh duffels for lots of trips, they were cheap, very light weight, pack down to nearly nothing empty. I have three big mesh duffles like that.
Some people carry their panniers onto the train, but some stations hold you to the published limit on number of pieces of luggage, so it is a good idea to be ready for that.